Ras protein, a guanosine triphosphatase (GTPase), is a molecular switch in signal transduction pathways that control cell growth and differentiation. In human cells, point mutations in crucial regions of Ras can cause uncontrolled cell growth, or tumors. We are trying to solve the structure of the complex between active Ras protein (Ras-GMPPNP) and the Ras interacting domain of RalGDS (RalGDS-RID), a GDP dissociation stimulator of Ral protein (Ras like protein). Our lab has solved the active Ras structure and the structure of RalGDS-RID with the MAD method. We would like to obtain a co-crystal data set, which will allow us to solve the structure of the complex by the Molecular Replacement method.